Questions about my campaign.

Hello all, 

I have have been writing for my homebrew campaign and have run into a road block. I can seem to write in this next town, could you help? 

Context: I have the party stumblibg across a mindflayer colony. Not everyone in the village is a mindflayer, but a lot are, and they are hiding by all casting Modify Memory. So the people in the village have no idea there are a bunch of mindflayers. I want to trick my players into believing it is a false hydra but it's actually a colony of mindflayer. How should I go about this?

The easy tropes of the false hydra are unexplained empty houses that don't look abandoned but all the townsfolk claim have always been empty, things like that. The false hydra is a big enough meme that any player well-versed in the D&D community will probably suspect that's what you're doing pretty quickly.

The only question then, is how do you subvert their expectations? I think you need a solid reason for why the mindflayers are doing this. Subtlety is usually not their MO, so why are these illithid hiding instead of just enslaving the whole town immediately? Once you have that reason, you can explain the empty houses, the missing people, etc.

Quote from BaconBreadman >>

Subtlety is usually not their MO, so why are these illithid hiding instead of just enslaving the whole town immediately?

 maybe the reason could be that there is something nearby that they want to get into, like a castle, and the village goes there sometimes, and the mindflayers want to sneak in.

Here's my working plan: In the campaign there is a militaristic, warmongering, society made up of giants & their kin. With established, years before the campaign there was a failed illithed invasion, where the Giants kicked out the illithed and put them into hiding. The illithed stayed in hiding so that they could recoup. So this would be one of the secret hideouts of the illithed. 

But I just can't seem to find a good plot hook for them, I don't want them to just kill everyone, but I want the illithed village to be more of a black market of information. They could also help them join society again (?).